EP3611832A1 - Photovoltaic inverter and method for operating the same - Google Patents

Abstract

The invention relates to a photovoltaic inverter (2) with a DC input (3) for connection to a DC source (4), a DC-DC converter (9), an intermediate circuit (10), a DC-AC Converter (11), an AC isolator (8), an AC connection (5) for connection to a supply network (6) and possibly a consumer (7), a battery stage (16), and a battery connection ( 14) for connection to a backup battery (15) and a method for operating such a photovoltaic inverter (2). According to the invention, a switching power supply (18) is provided, which switching power supply (18) is connected on the input side to the AC connection (5) and on the output side to the battery connection (14), so that when the DC source (4) is deactivated, the intermediate circuit (10) can be charged to a threshold value via the switching power supply (18) and the battery stage (16), whereupon the AC disconnector (8) can be actuated.

Description

The invention relates to a photovoltaic inverter with a DC input for connection to a DC source, a DC-DC converter, an intermediate circuit, a DC-AC converter, an AC isolator, an AC connection for connection to a Supply network and possibly a consumer, a battery level, and a battery connection for connection to a backup battery.

The invention further relates to a method for operating a photovoltaic inverter, the photovoltaic inverter having a DC input for connection to a DC source, a DC-DC converter, an intermediate circuit, a DC-AC converter, an AC Disconnector, an AC connection for connection to a supply network and possibly a consumer, a battery stage, and a battery connection for connection to a backup battery.

For example, the WO 2008/138020 A1 Such a hybrid inverter, which can be connected to a supply network, as well as operated as an island inverter to supply consumers with electrical energy, and a method for its control.

In order to be able to connect an inverter to a supply grid, the intermediate circuit voltage must be raised to the grid peak voltage before the AC isolator is closed. This is done by converting the voltage of the DC source (solar module or backup battery) accordingly. If the solar modules or the backup battery do not supply any voltage, it is usually not possible to raise the intermediate circuit voltage.

One possibility is a special connection of filter capacitors, so that they can be used as series capacitors for charging the intermediate circuit via the DC-AC converter. A disadvantage of this variant, however, is that additional leakage currents can result via the solar modules. In addition, the circuit complexity is increased.

Another option is to use a separate one Circuit for uploading the DC link voltage without a DC source from the supply network. However, this solution also requires space on the circuit board of the inverter and causes additional costs.

For the reasons mentioned, in some photovoltaic inverters, there is also no uploading from the supply network in the absence of a DC source or backup battery, which, however, worsens the energy management of the photovoltaic inverter.

The object of the present invention is to provide an above-mentioned photovoltaic inverter and a method for operating such a photovoltaic inverter so that the photovoltaic inverter can be started at any time and regardless of the presence of a voltage of the DC source, for example during twilight or night hours. The circuitry and the additional manufacturing costs for this should be as low as possible.

The object according to the invention is achieved by an above-mentioned inverter, wherein a switching power supply is provided, which switching power supply is connected on the input side to the AC connection and on the output side to the battery connection, so that when the DC source is deactivated, the intermediate circuit via the switching power supply and the battery Level can be charged to a threshold value, whereupon the AC isolator can be actuated. According to the invention, it is therefore provided that when the DC source is deactivated or inactive, that is to say if the solar modules, for example, do not deliver any energy during twilight or at night or because of snow, or the DC source is separated from the photovoltaic inverter, the intermediate circuit is connected via the Switching power supply and the battery level is charged until the voltage at the intermediate circuit necessary for connecting the photovoltaic solar inverter to the supply network is reached. The photovoltaic solar inverter is then connected to the supply network by actuating or closing the AC isolator by closing the AC isolator. Thereafter, the buffer battery can either be charged via the supply network or energy can be fed into the supply network from the buffer battery.

Reactive power can also be compensated for by the photovoltaic solar inverter. The circuitry complexity is low, since the existing battery level can also be used for this function and no separate upload circuit is required, which would require corresponding space on the circuit board of the photovoltaic inverter. No current measurement is therefore required for the voltage-controlled charging of the DC link. Any existing current sensor for measuring the higher charge / discharge currents of the backup battery would be too imprecise for these purposes.

A diode is advantageously arranged on the output side of the switching power supply. This diode between the switching power supply and the battery stage prevents current flow from the battery stage to the switching power supply.

A battery switch is advantageously provided for switching the buffer battery on and off. With such a battery switch, the backup battery can only be switched on when required, when this energy is used to supply consumers or when it is to be charged.

According to a further feature of the invention, a control device is provided which controls and activates or deactivates all components of the photovoltaic solar inverter.

The battery stage is preferably formed by a bidirectional DC-DC converter, in particular by a step-up converter with synchronous rectification with two semiconductor switches. Such a bidirectional DC-DC converter or step-up converter can be operated in both directions, i.e. for charging the buffer battery by converting the intermediate circuit voltage to the desired battery charging voltage and for converting the voltage provided by the buffer battery to the desired intermediate circuit voltage for supplying the consumers with electrical energy or for feeding energy from the backup battery into the supply or stand-alone grid.

The battery stage is advantageously connected to the control device, which control device for charging the intermediate circuit is formed with an essentially constant power draw from the switching power supply, in that the semiconductor switches of the step-up converter can be controlled with a predetermined duty cycle. In order not to overload the switched-mode power supply during the uploading of the intermediate circuit voltage, the semiconductor switches of the battery stage designed as a step-up converter are controlled with a corresponding pulse duty factor, which ensures that the power drawn from the switched-mode power supply is essentially constant.

The control device is connected to a program memory, in which the respective regulation for the control of the semiconductor switches of the battery level is stored in order to be able to prevent an overload of the switching power supply. Corresponding formulas for the duty cycle of the control of the semiconductor switches of the battery level or corresponding tables can be stored in the program memory.

The object of the invention is also achieved by an above-mentioned method for operating a photovoltaic inverter, a switching power supply being supplied from the AC connection and, when the DC source is deactivated, used to charge the intermediate circuit via the battery level to a threshold value, after which the AC disconnector is operated. This results in optimized energy management even when the DC source of the photovoltaic system is deactivated by charging the intermediate circuit via the switching power supply and the battery level until the photovoltaic inverter can be connected to the supply network by actuating or closing the AC isolator. After the photovoltaic inverter has been switched on, the backup battery can be charged via the supply network or energy can be fed into the supply network from the backup battery. Reactive power can also be compensated for by the photovoltaic solar inverter. For further advantages that can be achieved in this way, reference is made to the above description of the photovoltaic solar inverter.

The buffer battery is preferably switched on and off via a battery switch.

According to a further feature of the invention, the intermediate circuit is charged with constant power via the switching power supply, in that the battery stage is formed by a bidirectional DC-DC converter, in particular a step-up converter with synchronous rectification with two semiconductor switches, which semiconductor switches are controlled via a corresponding pulse duty factor , By charging with constant power, it is taken into account that the switching power supply can only provide a limited power or voltage. This prevents overloading of the switching power supply.

Before the intermediate circuit is charged, the voltage at the intermediate circuit is advantageously measured via the switching power supply and used as the starting voltage, and the pulse duty factor for controlling the semiconductor switches of the battery stage is determined as a function of the starting voltage. This ensures that the switching power supply is not overloaded without a current measurement being necessary.

The photovoltaic inverter is advantageously connected by closing the AC isolator when the threshold value of the voltage of the intermediate circuit corresponds at least to the network peak voltage of the supply network. For a three-phase inverter, this corresponds to the phase-to-phase peak voltage.

Fig. 1

ein Blockschaltbild einer üblichen Photovoltaik-Anlage;

Fig. 2

ein Blockschaltbild einer Ausführungsform eines Photovoltaik-Wechselrichters gemäß der vorliegenden Erfindung; und

Fig. 3

ein Blockschaltbild einer Ausführungsform der Batterie-Stufe des erfindungsgemäßen Photovoltaik-Wechselrichters.

The invention is explained in more detail with reference to the accompanying drawings. In it show:

Fig. 1

a block diagram of a conventional photovoltaic system;

Fig. 2

a block diagram of an embodiment of a photovoltaic inverter according to the present invention; and

Fig. 3

a block diagram of an embodiment of the battery stage of the photovoltaic inverter according to the invention.

In Fig. 1 a block diagram of a conventional photovoltaic system 1 is shown schematically. The photovoltaic system 1 contains a photovoltaic inverter 2, which is preferably formed by an HF inverter. At the DC voltage input or DC input 3 of the photovoltaic inverter 2, the DC voltage source or DC source 4 is connected, which is preferably formed from one or more parallel and / or series-connected solar modules. The AC voltage output or AC output 5 of the photovoltaic inverter 2 is connected to the supply network 6 and, if necessary, consumers 7. For example, the consumers 7 are formed by a motor, a refrigerator, a radio, etc. Likewise, the consumer 7 can also represent a home supply. An AC isolator 8 is located in front of the AC output 5 in the photovoltaic inverter 2 in order to be able to separate it from the supply network 6 and from any consumers 7 or to connect the photovoltaic inverter 2 to the supply network 6 and the consumers 7 to be carried out only if sufficient energy is provided by the DC source 4. The photovoltaic inverter 2 has a DC-DC converter 9 on the input side, an intermediate circuit 10 and an DC-AC converter 11 on the output side. The individual components of the photovoltaic inverter 2 are usually connected to a control device 13 via a data bus 12. The control device 13 is formed, for example, by a microprocessor, a microcontroller or a computer. Corresponding control of the individual components, such as the DC-DC converter 9 or the DC-AC converter 11, in particular the switching elements arranged therein, and the AC isolator 8 can be carried out via the control device 13. For this purpose, the individual regulating or control sequences are stored in the control device 13 by means of corresponding software programs and / or data or characteristic curves.

The energy management of such a so-called grid-connected photovoltaic inverter 2 is optimized in such a way that it covers as much of its own energy consumption as possible or feeds it in depending on the remuneration. In addition, energy management can cover peak shaving. Of course, a plurality of photovoltaic inverters 2 can also be connected in parallel, as a result of which more energy is available and more power can be provided for operating the consumers 7 or for feeding into the supply network 6.

Fig. 2 shows a block diagram of a photovoltaic inverter 2 according to the invention, wherein a buffer battery 15 can be connected via a battery connection 14 in order to be able to supply at least selected consumers 7 with electrical energy even when the DC source 4 does not supply any energy, for example during twilight, at night when there is snow on the solar module, or when the solar module has been disconnected from the photovoltaic inverter 2 (for example due to maintenance work, game bite or theft). The buffer battery 15 is connected to the intermediate circuit 10 with the corresponding intermediate circuit capacitor C _ZK via a battery stage 16. A battery switch 17, which is formed, for example, by a relay, can be arranged between the battery connection 14 and the buffer battery 15. The battery switch 17 can be arranged in the photovoltaic inverter 2, in the buffer battery 15 or between the buffer battery 15 and the photovoltaic inverter 2. If the battery switch 17 is integrated in the photovoltaic inverter 2, then the switching power supply 18 is connected on the output side to the battery stage 16 instead of the battery connection 14. Such a photovoltaic inverter 2 is also referred to as a hybrid inverter. If the battery switch 17 is arranged internally in the photovoltaic inverter 2, the battery connection 14 is located in front of the battery switch 17, that is to say between the battery stage 16 and the battery switch 17. The battery switch 17 also serves to that the switching power supply 18 is not burdened by the components of the backup battery 15.

According to the invention, a switching power supply 18 is provided, which is connected on the input side to the AC output 5 or output of the AC-DC converter 11 of the photovoltaic inverter 2. On the output side, the switched-mode power supply 18 is connected to the battery connection 14 or output of the battery stage 16 via a diode 19, which prevents current flow in the direction of the switched-mode power supply 18. In the event of a deactivated DC source 4, the intermediate circuit 10 is charged via the switching power supply 18 via the battery stage 16 to a threshold value which is necessary in order to connect the photovoltaic solar inverter 2 to the supply network 6 and possibly to the consumers 7 To allow closing of the AC disconnector 8.

The individual components are correspondingly connected to a control device 13 (not shown) and can accordingly be activated or deactivated and / or controlled or regulated accordingly. The control device 13 also includes an internal and / or external energy management system. Likewise, the components of the photovoltaic inverter 2 and the control device 13 can be connected to the switched-mode power supply 18 and can accordingly be supplied with electrical energy. In this case, a switching power supply provided specifically for supplying the components of the photovoltaic inverter 2 could be dispensed with.

The DC source 4 can also be connected directly to the intermediate circuit 10 without a DC-DC converter 9. When the sun is shining, the DC source 4 supplies the appropriate energy and the intermediate circuit 10 of the photovoltaic inverter 2 is charged to a voltage of 600 V, for example. This enables electrical energy to be fed into the supply network 6 by appropriately activating the DC-AC converter 11 and closing the AC isolator 8, which is formed, for example, by a relay.

There may also be a requirement that the buffer battery 15 must also be charged. Accordingly, the battery stage 16 is also activated, the battery switch 17 is closed and the buffer battery 15 is charged via the intermediate circuit 10. If the buffer battery 15 is charged or the DC source 4 no longer provides energy, for example due to the twilight, the corresponding components can also be deactivated again. With such a photovoltaic system 1, energy can also be drawn from the buffer battery 15 during the night, which is supplied to the consumers 7 via the AC output 5.

If the backup battery 15 is empty, the consumers 7 must be supplied with electrical energy via the supply network 6. For various reasons (such as low electricity prices, severe weather, bad weather forecast, ...), there may be a requirement that the backup battery 15 should also be deactivated Charge DC source 4 via the supply network 6.

For this purpose, the DC-AC converter 11 must be activated accordingly and the AC isolator 8 must be actuated or closed, for which purpose the voltage U _{ZK of} the intermediate circuit 10 must be charged to a corresponding threshold value. In such cases in which the DC source 4 is deactivated, the intermediate circuit 10 must first be charged in order to enable the photovoltaic inverter 2 to be connected to the supply network 6 by closing the AC isolator 8. According to the invention, this takes place via the battery stage 16, which is supplied via the switching power supply 18 from the AC output 5 of the photovoltaic inverter 2.

If the backup battery 15 is discharged, the battery switch 17 is opened, that is to say the backup battery 15 is separated from the photovoltaic inverter 2.

Since the switched-mode power supply 18 can only provide a limited power / voltage, it is uploaded accordingly by the battery stage 16, which is preferably designed as a bidirectional DC-DC converter, in particular as a step-up converter or booster. It is also taken into account that the switching power supply 18 is not overloaded and the battery switch 17 is open. In order to avoid overloading the switched-mode power supply 18, the battery stage 16 is activated accordingly when the intermediate circuit voltage U _{ZK is} uploaded. For this purpose, the battery stage 16 is connected to the control device 13 of the photovoltaic inverter 2. The corresponding control regulations can be stored in a program memory 20 connected to the control device 13.

The battery level 16 is corresponding, for example Fig. 3 designed as a step-up converter with synchronous rectification. The control of the semiconductor switch, the low-side switch 21 and high-side switch 22 takes place via a pulse duty factor D (t). The low-side switch 21 is driven with the duty cycle D (t) and the highside switch 22 with the inverted duty cycle 1-D (t). The duty cycle D (t) is controlled accordingly so that the current I supplied by the switching power supply 18 via the inductance L1 remains the same on average.

The pulse duty factor D (t) is selected such that an essentially constant and acceptable (in relation to the overload of the switching power supply 18) power / energy is drawn from the switching power supply 18 - for example a few watts (3W). In order to ensure this, the energy or voltage U _ZK in the intermediate circuit 10 must increase from an existing initial value of the intermediate circuit voltage U _ZK , the start value, to the desired final value, for example the network peak voltage, according to a predetermined function. For example, the duty cycle is calculated with D (t) = 1 - (U _SNT / U _{ZK, soll} (t)), USNT being the output voltage of the switching power supply 18 or the voltage at the intermediate circuit 10 at the start of the upload, and U _ZK (t) represent the time-dependent target value of the DC link voltage. The intermediate circuit voltage U _ZK (t) as a function of time t increases after a root function, according to which the voltage rise is more pronounced at the beginning and increases more gently with increasing time. The longer the intermediate circuit 10 is charged, the more the current flowing into the intermediate circuit 10 is reduced.

In the first step, the actual value of the intermediate circuit voltage U _{ZK is} measured to determine the duty cycle D (t) and defined as the starting value. As a result, the initial energy of the intermediate circuit 10 is known. The output voltage U _{SNT of} the switching power supply 18 is constant and is, for example, 130V.

After only constant power is drawn from the switching power supply 18, the energy / voltage of the intermediate circuit 10 increases according to the initial energy plus the energy supplied. For example, the sampling rate is 1 ms (1 kHz), so that energy is supplied cyclically every ms. According to this supplied energy, there is an increase in the intermediate circuit voltage U _ZK . Since the energy supplied is constant, the intermediate circuit voltage U _{ZK is} determined and does not have to be measured continuously. The pulse duty factor D (t) is continuously determined according to the above formula, for example every ms and the semiconductor switches 21 and 22 of the battery stage 16 are activated accordingly. The desired intermediate circuit voltage U _ZK (t), which corresponds at least to the network peak voltage or the external conductor peak voltage and from which Topology of the photovoltaic inverter 2 depends, is therefore reached after a short time, for example a few minutes (for example 2 minutes). The AC isolator 8 can then be closed and the DC-AC converter 11 activated, so that the buffer battery 15 is charged by the supply network 6 via the intermediate circuit 10.

The present photovoltaic inverter 2 and the method for operating the same enables connection to the supply network 6 or the consumers 7 even when the DC source 4 does not supply any energy, so that the buffer battery 15 can be charged via the supply network 6 or by the buffer battery 15 energy can be fed into the supply network 6. Likewise, 2 reactive power can be compensated by the photovoltaic inverter. The effort for this function that improves energy management is particularly low since already existing components of the photovoltaic inverter 2 are used.

Claims (13)

Photovoltaic inverter (2) with a DC input (3) for connection to a DC source (4), a DC-DC converter (9), an intermediate circuit (10), a DC-AC converter (11) , an AC isolator (8), an AC connection (5) for connection to a supply network (6) and possibly a consumer (7), a battery stage (16), and a battery connection (14) for connection with a backup battery (15), characterized in that a switching power supply (18) is provided, which switching power supply (18) is connected on the input side to the AC connection (5) and on the output side to the battery connection (14), so that when DC is deactivated -Source (4) of the intermediate circuit (10) via the switching power supply (18) and the battery stage (16) can be charged to a threshold value, after which the AC isolator (8) can be actuated. Photovoltaic inverter (2) according to claim 1, characterized in that a diode (19) is arranged on the output side of the switching power supply (18). Photovoltaic inverter (2) according to claim 1 or 2, characterized in that a battery switch (17) is provided for switching the buffer battery (15) on and off. Photovoltaic inverter (2) according to one of claims 1 to 3, characterized in that a control device (13) is provided. Photovoltaic inverter (2) according to one of claims 1 to 4, characterized in that the battery stage (16) is formed by a bidirectional DC-DC converter. Photovoltaic inverter (2) according to claim 5, characterized in that the battery stage (16) is formed by a step-up converter with synchronous rectification with two semiconductor switches (21, 22). Photovoltaic inverter (2) according to claim 6, characterized in that the battery stage (16) with the control device (13), which is designed to charge the intermediate circuit (10) with substantially constant power consumption from the switching power supply (18), in that the semiconductor switches (21, 22) of the step-up converter can be controlled with a predetermined pulse duty factor (D (t)) , Photovoltaic inverter (2) according to claim 7, characterized in that the control device (13) is connected to a program memory (20). Method for operating a photovoltaic inverter (2), the photovoltaic inverter (2) having a DC input (3) for connection to a DC source (4), a DC-DC converter (9), an intermediate circuit ( 10), a DC-AC converter (11), an AC isolator (8), an AC connection (5) for connection to a supply network (6) and possibly a consumer (7), a battery stage (16 ), and has a battery connection (14) for connection to a backup battery (15), characterized in that a switching power supply (18) is supplied by the AC connection (5) and, when the DC source (4) is deactivated, for charging the The intermediate circuit (10) is used to a threshold value via the battery stage (16), whereupon the AC isolator (8) is actuated. A method according to claim 9, characterized in that the buffer battery (15) is switched on and off via a battery switch (17). A method according to claim 9 or 10, characterized in that the intermediate circuit (10) is charged with constant power via the switching power supply (18) by the battery stage (16) with a bidirectional DC-DC converter, in particular step-up converter with synchronous rectification two semiconductor switches (21, 22) are formed, which semiconductor switches (21, 22) are controlled via a corresponding pulse duty factor (D (t)). A method according to claim 11, characterized in that before charging the intermediate circuit (10) via the switching power supply (18) the voltage (U _ZK ) at the intermediate circuit (10) is measured and as the starting voltage is used, and the duty cycle (D (t)) is determined as a function of the starting voltage. Method according to one of claims 9 to 12, characterized in that the AC isolator (8) is actuated when the threshold value of the voltage (U _ZK ) of the intermediate circuit (10) corresponds at least to the network peak voltage of the supply network (6).

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